Wetting phenomena are ubiquitous in nature and technology. A solid substrate exposed to the environment is almost invariably covered by a layer of fluid material. In this review, the surface forces that lead to wetting are considered, and the equilibrium surface coverage of a substrate in contact with a drop of liquid. Depending on the nature of the surface forces involved, different scenarios for wetting phase transitions are possible; recent progress allows us to relate the critical exponents directly to the nature of the surface forces which lead to the different wetting scenarios. Thermal fluctuation effects, which can be greatly enhanced for wetting of geometrically or chemically structured substrates, and are much stronger in colloidal suspensions, modify the adsorption singularities. Macroscopic descriptions and microscopic theories have been developed to understand and predict wetting behavior relevant to microfluidics and nanofluidics applications. Then the dynamics of wetting is examined. A drop, placed on a substrate which it wets, spreads out to form a film. Conversely, a nonwetted substrate previously covered by a film dewets upon an appropriate change of system parameters. The hydrodynamics of both wetting and dewetting is influenced by the presence of the three-phase contact line separating "wet" regions from those that are either dry or covered by a microscopic film only. Recent theoretical, experimental, and numerical progress in the description of moving contact line dynamics are reviewed, and its relation to the thermodynamics of wetting is explored. In addition, recent progress on rough surfaces is surveyed. The anchoring of contact lines and contact angle hysteresis are explored resulting from surface inhomogeneities. Further, new ways to mold wetting characteristics according to technological constraints are discussed, for example, the use of patterned surfaces, surfactants, or complex fluids.

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Wetting and Spreading

Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

The speed at which a liquid can move over a solid surface is strongly limited when a three-phase contact line is present, separating wet from dry regions. When enforcing large contact line speeds, this leads to the entrainment of drops, films, or air bubbles. In this review, we discuss experimental and theoretical progress revealing the physical mechanisms behind these dynamical wetting transitions. In this context, we discuss microscopic processes that have been proposed to resolve the moving–contact line paradox and identify the different dynamical regimes of contact line motion.

http://stilton.tnw.utwente.nl/people/snoeijer/Papers/2013/SnoeijerARFM13.pdf

Moving Contact Lines: Scales, Regimes, and Dynamical Transitions

Surface instabilities in soft matter have been the subject of increasingly innovative research aimed at better understanding the physics of their formation and their utility in patterning, organizing, and measuring materials properties on the micro and nanoscale. The focus of this Review is on a type of instability pattern known as surface wrinkling, covering the general concepts of this phenomenon and several recent applications involving the measurement of thin-film properties. The ability of surface wrinkling to yield new insights into particularly challenging materials systems such as ultrathin films, polymer brushes, polyelectrolyte multilayer assemblies, ultrasoft materials, and nanoscale structured materials is highlighted. A perspective on the future directions of this maturing field, including the prospects for advanced thin-film metrology methods, facile surface patterning, and the control of topology-sensitive phenomena, such as wetting and adhesion, is also presented.

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Surface Wrinkling: A Versatile Platform for Measuring Thin‐Film Properties

to develop synthetic analogs as“smart” adhesives, that is, where the geometry of the patternscan tailor the adhesion of the material. However, there arecurrently two limitations in the design of patterned adhesives.First, because of either materials selection or geometric de-sign, the adhesive properties are not always reversible overthe course of multiple attachment–detachment cycles.

Surface wrinkles for smart adhesion

Residual stresses in thin polymer films cause rupture and dominate early stages of dewetting

Residual stresses in thin polymer films cause rupture and dominate early stages of dewetting.

Abstract.We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.

/pdf/dewetting-of-thin-polymer-films-2o78631xky.pdf

Dewetting of Thin Polymer Films

Adhesion at polydimethylsiloxane (PDMS)-acrylic adhesive interfaces is shown to be enhanced through micropatterning of the PDMS substrate. By varying the geometry of the patterns (groves and hexagonal arrays of pillars of micrometer sizes, obtained through soft lithography techniques) and comparing rigid and deformable substrates, the respective roles of the geometry and the size and flexibility of the pattern features on the level of adhesion have been analyzed. For cylindrical pillars, two regimes are clearly identified: for a relatively low aspect ratio (h/r 1.6.; for a high aspect ratio (h/r > 3), only rigid patterned substrates enhance adhesion, then the only possible contribution to energy dissipation comes from the enhanced viscoelastic losses associated with the pattern that induce modifications of the strain field within the adhesive layer. Soft, high aspect ratio patterns lose their efficiency even if still bent under the effect of the peel forces. This is because when bent, some of the pillars touch each other and remain stuck together, lying flat on the surface after the passage of the peel front. The bending elastic energy of the pillars (which is still lost) is then balanced by the corresponding gain in surface energy of the substrate in the peeled region. These systematic experiments demonstrate that the ability of the patterned surface to be deformed plays a crucial role in enhancing adhesion and allow us to propose a way to fine tune the level of adhesion at PDMS-acrylic adhesive interfaces, independently of the chemistry of the adhesive.

Adhesion enhancement through micropatterning at polydimethylsiloxane-acrylic adhesive interfaces.

Dewetting of thin polystyrene films deposited onto silicon wafers exhibits unusual dynamics and rim morphologies. Here, we present a new theoretical approach of these phenomena taking into account both the viscoelastic properties of the film and the non-zero velocity of the film at the interface with the substrate (i.e. slippage). We show how these two ingredients lead to: a) a very asymmetric shape of the rim as the film dewets; b) a decrease of the dewetting velocity with time like t−1/2 for times shorter than the reptation time (for larger times, the dewetting velocity reaches a constant value). Recent experiments by Damman, Baudelet and Reiter (Phys. Rev. Lett., 91 (2003) 216101) present, however, a much faster decrease of the dewetting velocity. We then show how this striking result may be explained by the presence of residual stresses in the film.

https://iopscience.iop.org/article/10.1209/epl/i2005-10312-y/pdf

Dewetting of thin viscoelastic polymer films on slippery substrates

We study the adhesion between a cross-linked elastomer and a flat solid surface where polymer chains have been end-grafted. To understand the adhesive feature of such a system, one has to study both the origin of the grafted layer interdigitation with the network, and the end-grafted chains extraction out of the elastomer when it comes unstuck from the solid surface. We shall tackle here the first aspect for which we develop a partial interdigitation model that lets us analytically predict a critical surface grafting density $\sigma^{*} \simeq P^{{1/10}}N^{-{3/5}}$ beyond which the layer no longer interdigitates with the elastomer. We then relate this result with recent adhesion measurements.

Thomas Vilmin

Papers

Residual stresses in thin polymer films cause rupture and dominate early stages of dewetting.

Dewetting of Thin Polymer Films

Adhesion enhancement through micropatterning at polydimethylsiloxane-acrylic adhesive interfaces.

Dewetting of thin viscoelastic polymer films on slippery substrates

Interdigitation between surface-anchored polymer chains and an elastomer : consequences for adhesion promotion